Turnover time of the non-structural carbohydrate pool influences δ18O of leaf cellulose

Certainty regarding the degree to which organic molecules exchange oxygen with local water during plant cellulose synthesis (p(ex)) is necessary for cellulose oxygen isotope (δ(18)O(cell))-based applications in environmental and ecological studies. However, the currently accepted notion that p(ex) is a constant of ca. 0.42 appears inconsistent with biochemical theory, which predicts that marked variation may be present in p(ex), in relation to variation in the turnover time (τ) of the carbohydrate pool available for cellulose synthesis. The above prediction was tested in the present study with the analysis of data collected from leaves of Ricinus communis grown in controlled environmental conditions that varied in light intensity and vapour pressure deficit. The results revealed the existence of considerable variation in both p(ex) and τ across plants in the various growth environments. Moreover, despite uncertainties in estimates of the proportion of source water in the synthesis water (p(x)) and of the biochemical fractionation factor (ε(o)), our experiment yielded strong evidence that p(ex) exhibits a significant, positive relationship with τ, consistent with biochemical theory. The observed variation in p(ex) in association with τ has important implications for the interpretation of δ(18)O(cell) data in environmental/ecological studies.